The Shore pyroscope, Fig. 17, operates upon a principle very similar to that of the Fery optical pyrometer. The instrument has a scale graduated to read temperatures directly, which is a material advantage. The design of optical parts is rather un-
FIG. 16.—Fery optical pyrometer.
FIG. 17.—Shore pyroscope.
necessarily complicated and it is difficult to match the two fields on account of color differences.
Wanner Pyrometer.—Figure 18 illustrates the arrangement of the optical parts in the Wanner pyrometer. The comparison light is a six-volt incandescent lamp illuminating a glass matt surface in front of the slit /S2. The slit Si is illuminated by the source sighted upon. Light from each slit passes through the
FIG. 18.—Wanner pyrometer.
collimating lens Oi, the direct vision spectroscope P, a Wollaston prism R, a bi-prism J5, the second collimating lens $2, and is brought to a focus at F. The Wollaston prism produces two images of each slit which are polarized at right angles to each other. The bi-prism again doubles the number of images so so that there are finally four images of each slit at F. Six of these images are dia-phragmed off by the screen D. The two remaining images, one of each slit, are superposed and are polarized at right angles to each other. From this point the light passes through the nicol prism A and the ocular lens E. The direct vision spectroscope is so adjusted that only red light of wave length about >- = 0.65ju reaches the eye, the other colors being diaphragmed off by the screen D. The ocular is focused on the dividing edge of the bi-prism B. The eye perceives a circular photometric field half of which is illuminated by the slit Si and half by the slit 82. The light from the two fields is plane polarized and the plane of polarization in one field is at right angles to the plane of polarization in the other field. Consequently on rotating the nicol